The present invention relates to a method for producing a worm shaft for use in a power steering apparatus provided with a rotation control valve.
In a power steering apparatus of this type, a rotation control valve is constituted by a valve rotor, which rotates together with an input shaft, and a valve sleeve, which is integrated with a worm shaft connected to the input shaft through a torsion bar. The rotation control valve controls, via fluid control passages formed therein and in the inner surface of an enlarged-diameter portion of the valve sleeve, the distribution of pressurized fluid to a piston threadedly engaged with the worm shaft to thereby provide steering power to an output shaft engaged with the piston. In the rotation control valve, the valve rotor and the valve sleeve are maintained in the neutral state during nonoperational periods by the torque of the torsion bar, and relative rotation of the valve rotor and the valve sleeve is restricted such that the input shaft and the worm shaft can rotate relatively to one another within a predetermined angular range defined by a fail safe structure formed therebetween. More specifically, interleaved grooves which allow relative rotation between the input shaft and the worm shaft are formed in the outer periphery of the input shaft and the inner periphery of the worm shaft, which structure is hereinafter referred to as a fail safe structure, thereby making possible relative rotation in the amount allowed by the gap between the grooves but making it possible to steer the vehicle if the power steering hydraulic system fails.
In the conventional worm shaft, the grooves of the fail safe structure and the fluid control passage are separately machined. It may happen that the neutral position of the valve sleeve and valve rotor does not occur at a point where equal spaces are present on either side of each of the grooves of the fail safe structure due to a misalignment between the grooves and the fluid control passages. Such a power steering apparatus has a defect that a deterioration in strength due to metal fatigue may occur because excessive torque is applied to the torsion bar during rotation in one of the two directions for which the amount of possible movement is greater. There is a further disadvantage that the manufacture of this product is expensive because two steps are required, one for forming the grooves of the fail safe structure and the other for forming the fluid control passages.